There will be two types of trail-O available: Pre-O Sprint on day 1 and Pre-O on day(s) 2 and/or 3. Both will be close to the arena on each day. To be confirmed. Trail-O starts will be timed to allow you to take part after completing the Sprint or the Long if you so wish.
Pre-O completely eliminates the element of speed over the ground, but makes the map interpretation element much harder. Depending on the level of difficulty, up to five control markers are hung at each site and only one will correspond exactly with the control description and control circle position. Indeed, (except on Novice courses), one possible answer is 'None of the controls corresponds'.
The full explanation of Trail-O is at https://www.britishorienteering.org.uk/site/trailo